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Posted: Jul 30, 2014

Less to the Milky Way than previously thought

(Nanowerk News) The Milky Way is less massive than astronomers previously thought, according to new research. For the first time, scientists have been able to precisely measure the mass of the galaxy that contains our Solar system. A team led by researchers at the University of Edinburgh have found that the Milky Way is approximately half the mass of a neighbouring galaxy – known as Andromeda – which has a similar structure to our own. They publish their results in the journal Monthly Notices of the Royal Astronomical Society ("Balancing mass and momentum in the Local Group").

An image of the Andromeda Galaxy, M31. (Image: Adam Evans)

The Milky Way and Andromeda are the two largest members of a cluster of galaxies which astronomers call the Local Group. Both galaxies have a spiral shape and appear to be of similar dimensions, but until now scientists had been unable to prove which is most massive as previous studies were only able to measure the mass enclosed within both galaxies’ inner regions.

The Edinburgh astronomers used recently published data on the known distances between galaxies – as well as their velocities – to calculate the total masses of Andromeda and the Milky Way. Revealing this for both galaxies, they also found that so-called ‘dark’ matter makes up 90% of the matter in both systems.

Dark matter is a little understood invisible substance which makes up most of the outer regions of galaxies and around 27% of the content of the Universe. The researchers estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be about twice as massive in total. Their work should help astronomers learn more about how the outer regions of galaxies are structured.

Dr Jorge Peñarrubia, of the University of Edinburgh’s School of Physics and Astronomy, who led the study, said: “We always suspected that Andromeda is more massive than the Milky Way, but weighing both galaxies simultaneously proved to be extremely challenging. Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible.”